Process for Producing Die-Cast Parts

ABSTRACT

In a process for producing die-cast parts made of an aluminum alloy, the aluminum alloy is exposed to high shearing forces in a mixing and kneading machine, is removed as partially solid aluminum alloy with a predefined solids content, is transferred into a filling chamber of a die-casting machine and is introduced into a casting mold using a piston, wherein a solids content of the aluminum alloy in a working space of the mixing and kneading machine is set to a predefined solids content by cooling and heating the working space in a targeted manner.

The invention relates to a process for producing die-cast parts made ofan aluminum alloy.

Die-cast parts made of aluminum alloys are being used ever morefrequently, inter alia, in the automotive industry for reasons of anincreasing demand for weight reduction. For casting technology reasons,it is generally the case that a cast part wall thickness of about 2 mmcannot be undershot, for example in the case of nodes for space framestructures, with conventional die-casting processes. The filling of thedie-casting mold with partially solid metal melts by using thixocastingor rheocasting leads to better filling of the mold and, as a result, toa possible further reduction in the cast part wall thickness to about 1mm. As the wall thickness decreases, however, the reducedforce-absorption capability increasingly becomes a limiting factor. Thisdisadvantage by itself could be countered by the addition ofnanoparticles to an aluminum alloy matrix. However, there is a lack ofsuitable processes for cost-effectively producing aluminum alloysreinforced with nanoscale particles and for the preparation thereof toform partially solid metal melts for die casting.

The invention is based on the object of providing a process of the typementioned in the introduction, with which process a partially solidaluminum alloy melt can be provided continuously in a cost-effectivemanner and further processed to form die-cast parts. It is a furtherobject of the invention to provide a process for producing die-castparts which are reinforced with nanoparticles and are made of analuminum alloy, with which process a partially solid aluminum alloy meltcan be provided continuously in a cost-effective manner under the actionof shearing forces typical of the process with a high fine dispersion ofnanoparticles and further processed to form die-cast parts.

The first object is achieved according to the invention in that thealuminum alloy is exposed to high shearing forces in a mixing andkneading machine, having a housing with a working space, which issurrounded by an inner housing sleeve, and a worm shaft, which rotatesabout a longitudinal axis and moves to and fro translationally in thelongitudinal axis in the inner housing sleeve and is provided withkneading blades, and with kneading bolts, which are fastened to theinner housing sleeve and protrude into the working space, wherein liquidaluminum alloy is fed to the working space at one end of the housingand, at the other end of the housing, is removed from the working spaceas partially solid aluminum alloy with a predefined solids content, istransferred into a filling chamber of a die-casting machine and isintroduced into a casting mold by means of a piston, wherein the solidscontent of the aluminum alloy in the working space is set to thepredefined solids content by cooling and heating the working space in atargeted manner. Here, the high shearing forces present in the kneadingprocess in the partially solidified phase state continuously comminutedendritic branches which form, and this leads to an increased ductilityof the die-cast parts. The high compression forces additionally lead toa greater transfer of heat, which ultimately makes it possible to setthe solids content in the aluminum alloy more precisely.

The second object is achieved according to the invention in thatnanoparticles are mixed with the aluminum alloy and finely dispersed inthe aluminum alloy by high shearing forces in a mixing and kneadingmachine, having a housing with a working space, which is surrounded byan inner housing sleeve, and a worm shaft, which rotates about alongitudinal axis and moves to and fro translationally in thelongitudinal axis in the inner housing sleeve and is provided withkneading blades, and with kneading bolts, which are fastened to theinner housing sleeve and protrude into the working space, wherein liquidaluminum alloy and nanoparticles are fed to the working space at one endof the housing and, at the other end of the housing, are removed fromthe working space as partially solid aluminum alloy with a predefinedsolids content and with nanoparticles finely dispersed in the aluminumalloy, are transferred into a filling chamber of a die-casting machineand are introduced into a casting mold by means of a piston, wherein thesolids content of the aluminum alloy in the working space is set to thepredefined solids content by cooling and heating the working space in atargeted manner. Here, in addition to the comminution of dendriticbranches which form and the resultant higher ductility, the highshearing forces present in the kneading process in the partiallysolidified phase state finely disperse the nanoparticles, which isrequired for the strength-increasing effect thereof.

It is expedient that the inner housing sleeve is surrounded by an outerhousing sleeve such that an intermediate space preferably in the form ofa hollow cylinder is formed, and cold and/or hot gases are conductedthrough the intermediate space for cooling and heating the workingspace. Air, preferably compressed air, is preferably conducted throughthe intermediate space for cooling, and hot gases, preferably combustiongases, are preferably conducted through the intermediate space forheating.

The gases are preferably conducted through the intermediate space incountercurrent to the direction in which the aluminum alloy istransported.

The solids content of the aluminum alloy is preferably set to 40 to 80%,in particular to more than 50%.

In a preferred embodiment of the process according to the invention, thepartially solid aluminum alloy is removed from the working space as apartially solid metal strand. The continuously emerging, partially solidmetal strand is split into partially solid metal portions and thepartially solid metal portions are transferred into the filling chamberof the die-casting machine.

The content of the nanoparticles in the alloy is preferably betweenabout 0.1 and 10% by weight. Suitable, cost-effective nanoparticlesconsist preferably of fumed silica, such as e.g. Aerosil®. However, itis also possible to use other nanoparticles, such as e.g. the knowncarbon nanotubes (CNT) and also further, nanoscale particles which areproduced, for example, by the known Aerosil® process and are made ofmetal and semimetal oxides, such as e.g. aluminum oxide (Al₂O₃),titanium dioxide (TiO₂), zirconium oxide (ZrO₂), antimony(III) oxide,chromium(III) oxide, iron(III) oxide, germanium(IV) oxide, vanadium(V)oxide or tungsten(VI) oxide.

Further advantages, features and details of the invention will becomeapparent from the following description of preferred exemplaryembodiments and with reference to the drawing, which serves merely forelucidation and is not to be interpreted as having a limiting effect.Schematically, in the drawing,

FIG. 1 shows a longitudinal section through a die-casting machine withan upstream mixing and kneading machine;

FIG. 2 shows a longitudinal section through part of a mixing andkneading machine;

FIG. 3 shows a cross section through the mixing and kneading machineshown in FIG. 1;

FIG. 4 shows characteristic shearing and stretching flow fields in aproduct mass, triggered by a kneading blade moving past a kneading bolt;

FIG. 5 shows the continuous production of partially solid startingmaterial for die casting with an arrangement according to FIG. 1.

A plant, shown in FIG. 1, for die casting die-cast parts which areoptionally reinforced with nanoparticles and are made of an aluminumalloy has a die-casting machine 10 and a mixing and kneading machine 30upstream of the die-casting machine 10.

The die-casting machine 10, which is shown only in part in the drawing,is a commercially available machine for conventionally die castingaluminum alloys and has, inter alia, a filling chamber 12, which isconnected to a stationary side 18 of a casting mold, with an opening 16for receiving the metal which is to be ejected from the filling chamber12 and introduced into a mold cavity 14 of the casting mold by means ofa piston 20.

The mixing and kneading machine 30 is shown in detail in FIGS. 2 and 3.The basic design of such a mixing and kneading machine is known, forexample, from CH-A-278 575. The mixing and kneading machine 30 has ahousing 31 with a working space 34, which is surrounded by an innerhousing sleeve 32 and in which there is arranged a worm shaft 36, whichrotates about a longitudinal axis x and moves to and fro translationallyin the longitudinal axis x in the inner housing sleeve 32. The wormshaft 36 is interrupted in the circumferential direction such thatindividual kneading blades 38 are formed. Axial through openings 40 arethereby formed between the individual kneading blades 38. Kneading bolts42 protrude from the inner side of the inner housing sleeve 32 into theworking space 34. The kneading bolts 42 on the housing side engage intothe axial through openings 40 of the kneading blades 38 arranged on themain or worm shaft 36. A drive shaft 44 arranged concentrically to theworm shaft 36 is guided out of the inner housing sleeve 32 at the endand is connected to a drive unit (not shown in the drawing) forexecuting a rotational movement of the worm shaft 36. A deviceinteracting with the worm shaft 36 for executing the translationalmovement of the worm shaft 36 is likewise not shown in the drawing.

The cylindrical inner housing sleeve 32 of the mixing and kneadingmachine 30, which delimits the working space 34, is surrounded by acylindrical outer housing sleeve 46. The inner housing sleeve 32 and theouter housing sleeve 46 form a dual sleeve and thereby enclose anintermediate space 48 in the form of a hollow cylinder.

An introduction opening 50 for feeding liquid aluminum alloy andoptionally nanoparticles into the working space 34 is provided at thatend of the housing 31 which is close to the drive side of the worm shaft36. Although only one introduction opening 50 is shown in the drawing,two separate introduction openings for the aluminum alloy and for thenanoparticles can be provided. In principle, it is also possible toadmix the nanoparticles with the liquid aluminum alloy even before themetal is introduced into the kneading and mixing machine 30. An outletopening 52 for removing partially solid aluminum alloy optionally withnanoparticles dispersed therein is provided at that end of the innerhousing sleeve 32 which is remote from the drive side of the worm shaft36.

Inlet openings 54, 56 for introducing cold or hot gases into theintermediate space 48 are provided in the outer housing sleeve 46 atthat end of the housing 31 which is remote from the drive side of theworm shaft 36. Correspondingly, outlet openings 58, 60 for the dischargeof the gases from the intermediate space 48 are provided at that end ofthe housing 31 which is close to the drive side of the worm shaft 36. Inorder to ensure a maximum throughflow of gas, which is distributeduniformly over the circumference of the inner housing sleeve 32, fromthe inlet openings 54, 56 to the outlet openings 58, 60, and thus auniform discharge of heat from the working space 34 or a uniformintroduction of heat into the working space 34, the inlet and outletopenings 54, 56 and 58, 60, respectively, are according to FIG. 3arranged distributed uniformly about the circumference of the outerhousing sleeve 46.

FIG. 4 shows, in a schematic illustration, characteristic shearing andstretching flow fields in a product mass P, as triggered by a kneadingblade 38 moving past a kneading bolt 42 in the case of a mixing andkneading machine 30 formed according to the prior art. The direction inwhich the kneading blade 38 rotates is indicated schematically by acurved arrow A, whereas the translational movement of the kneading blade38 is indicated by a double-headed arrow B. The rotational movement ofthe kneading blade 38 means that its tip splits the product mass P, asindicated by arrows C, D. There is a gap 41, the width of which variesdepending on the rotation and translational movement of the worm shaft36, between the kneading bolt 42 and the main face 39 of the kneadingblade 38, which faces toward the kneading bolt 42, and the kneadingblade 38 moving past the latter. A shearing process is brought about inthe product mass P in this gap 41, as indicated by arrow E. The productmass P expands and reorientates itself both upstream and downstream ofthe kneading bolt 42, as indicated by rotation arrows F, G. As alreadymentioned in the introduction, there is a maximum convergence of thekneading blade 38 and the kneading bolt 42 and thus a maximum shearingrate in the product mass P per shearing cycle owing to the sinusoidalaxial movement of the respective kneading blade 38 on a line.

In the text which follows, the mode of operation of the plant for diecasting die-cast parts which are optionally reinforced withnanoparticles and are made of an aluminum alloy is explained in moredetail, by way of example, with reference to FIGS. 1 and 2.

An aluminum alloy melt kept just above the liquidus temperature of thealloy is fed to the working space 34 in metered form alone or togetherwith nanoparticles via the introduction opening 50. The pinching of thepartially solidified aluminum alloy with nanoparticles between thekneading blades 38 and the kneading bolts 42 results in the applicationof high shearing forces, which both lead to the comminution of dendriticbranches and finely disperse the nanoparticles present in the form ofagglomerates. Efficient, homogenizing mixing results from thecombination of a radial and longitudinal mixing effect. By controllingthe flow of cold and hot gases through the intermediate space 48 betweenthe inner housing sleeve 32 and the outer housing sleeve 46, the solidscontent of the aluminum alloy in the working space 34 is set such thatit is in the desired range when the metal is removed through the outletopening 52.

The desired solids content of the aluminum alloy is set by measuring thechange in viscosity of the metal melt in the kneading and mixing machine30. The viscosity, which rises as the solids content of the partiallysolid aluminum alloy increases, can be determined, for example, bymeasuring the rotational resistance at the drive shaft 44 of the wormshaft 36. By determining the rotational resistance for defined solidscontents, it is possible to specify appropriate setpoint values, towhich measured actual values are regulated by controlling the flow ofcold and hot gases through the intermediate space 48 between the innerhousing sleeve 32 and the outer housing sleeve 46.

The aluminum alloy having the desired solids content and optionallycomprising finely dispersed nanoparticles is introduced via theintroduction opening 16 into the filling chamber 12 of the die-castingmachine 10, and is injected intermittently from the latter into the moldcavity 14 of the casting mold from the filling chamber 12 in a knownmanner by means of the piston 20.

With reference to FIG. 5, the text which follows provides a moredetailed explanation, by way of example, of the continuous production ofpartially solid, bar-shaped starting material for die casting die-castparts which are optionally reinforced with nanoparticles and are made ofan aluminum alloy. The mode of operation explained above with referenceto FIGS. 1 and 2 is retained.

The aluminum alloy having the desired solids content and optionallycomprising finely dispersed nanoparticles is continuously ejected viathe outlet opening 52 in the form of a partially solid metal strand 70.Partially solid metal portions 72 are cut to length from the partiallysolid metal strand 70, for example using a rotating blade. The partiallysolid metal portions 72 usually each correspond to the quantity of metalrequired for producing an individual die-cast part and, for each shot,are transferred individually into the filling chamber 12 of thedie-casting machine 10 and injected intermittently from the latter intothe mold cavity 14 of the casting mold from the filling chamber 12 in aknown manner by means of the piston 20.

The partially solid metal strand 70 usually leaves the mixing andkneading machine 30 in the direction of the longitudinal axis x of theworm shaft 36 in a horizontal direction, although another, e.g.vertical, outlet direction is also conceivable. The cross section of themetal strand 70 is determined by the cross section of the outlet opening52, and is usually circular. The partially solid metal portions 72 canbe grasped by tongs, for example, and transferred into the fillingchamber 12 of the die-casting machine 10.

1. A process for producing die-cast parts made of an aluminum alloy,wherein the aluminum alloy is exposed to high shearing forces in amixing and kneading machine, having a housing with a working space,which is surrounded by an inner housing sleeve, and a worm shaft, whichrotates about a longitudinal axis and moves to and fro translationallyin the longitudinal axis in the inner housing sleeve and is providedwith kneading blades, and with kneading bolts, which are fastened to theinner housing sleeve and protrude into the working space, wherein liquidaluminum alloy is fed to the working space at one end of the housingand, at the other end of the housing, is removed from the working spaceas partially solid aluminum alloy with a predefined solids content, istransferred into a filling chamber of a die-casting machine and isintroduced into a casting mold using a piston, wherein the solidscontent of the aluminum alloy in the working space is set to thepredefined solids content by cooling and heating the working space in atargeted manner.
 2. The process as claimed in claim 1, wherein the innerhousing sleeve is surrounded by an outer housing sleeve such that anintermediate space is formed, and cold and/or hot gases are conductedthrough the intermediate space for cooling and heating the workingspace.
 3. The process as claimed in claim 2, wherein air is conductedthrough the intermediate space for cooling, and hot gases are conductedthrough the intermediate space for heating.
 4. The process as claimed inclaim 2, wherein the gases are conducted through the intermediate spacein countercurrent to the direction in which the aluminum alloy istransported.
 5. The process as claimed in claim 1, wherein in order toset a desired solids content, the viscosity of the aluminum alloy in theworking space is measured and set to a predefined value by cooling andheating the working space in a targeted manner.
 6. The process asclaimed in claim 1, wherein the solids content of the aluminum alloy isset to 40 to 80%.
 7. The process as claimed in claim 1, wherein thepartially solid aluminum alloy is removed from the working space as apartially solid metal strand, the partially solid metal strand is splitinto partially solid metal portions and the partially solid metalportions are transferred into the filling chamber of the die-castingmachine.
 8. The process as claimed in claim 1, wherein in order toproduce die-cast parts reinforced with nanoparticles, nanoparticles aremixed with the aluminum alloy and finely dispersed in the aluminum alloyby high shearing forces in the mixing and kneading machine, whereinliquid aluminum alloy and nanoparticles are fed to the working space atone end of the housing and, at the other end of the housing, are removedfrom the working space as partially solid aluminum alloy with apredefined solids content and with nanoparticles finely dispersed in thealuminum alloy.
 9. The process as claimed in claim 8, wherein thecontent of the nanoparticles in the alloy is 0.1 to 10% by volume. 10.The process as claimed in claim 9, wherein the nanoparticles are fumedsilica, carbon nanotubes (CNT) or nanoscale particles of metal orsemimetal oxides.
 11. The process as claimed in claim 2, wherein theintermediate space is in the form of a hollow cylinder.
 12. The processas claimed in claim 3, wherein the air is compressed air and the hotgases are combustion gases.
 13. The process as claimed in claim 6,wherein the solids content of the aluminum alloy is set to 50 to 80%.14. The process as claimed in claim 10, wherein the nanoscale particlesof metal or semimetal oxides include aluminum oxide (Al₂O₃), titaniumdioxide (TiO₂), zirconium oxide (ZrO₂), antimony(III) oxide,chromium(III) oxide, iron(III) oxide, germanium(IV) oxide, vanadium(V)oxide or tungsten(VI) oxide.